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Unformatted text preview: CAP4730: Computational
Structures in Computer Graphics 3D Concepts Outline
• Basic Idea of 3D
• Projections
• What are some things we didn’t have to
worry about before?
• What are some new things we can do? Right Handed Coordinate System
+Y +Y +Z
+X
+Z +X Viewing a 3D world
+Y
We have a model in this world
and would like to view it from a
new position. +X
+Z
We’ll call this new position the
camera or eyepoint. Our job is to
figure out what the model looks
like on the display plane. Parallel Projection
+Y +Z +X Perspective Projection
+Y +Z +X What are some new things to
think about?
Hidden Surface Removal
Visibility
Depth Cueing How to make a 2D image appear as
3D!
• Output is typically 2D Images
• Yet we want to show a 3D world!
• How can we do this?
– We can include ‘cues’ in the image that give
our brain 3D information about the scene
– These cues are visual depth cues Visual Depth Cues
•
•
•
• Monoscopic Depth Cues (single 2D image)
Stereoscopic Depth Cues (two 2D images)
Motion Depth Cues (series of 2D images)
Physiological Depth Cues (body cues) Monoscopic Depth Cues
• Interposition
– An object that occludes another is closer • Shading
– Shape info. Shadows are included here • Size
– Usually, the larger object is closer • Linear Perspective
– parallel lines converge at a single point • Surface Texture Gradient
– more detail for closer objects • Height in the visual field
– Higher the object is (vertically), the further it
is • Atmospheric effects
– further away objects are blurrier • Brightness
– further away objects are dimmer Stereoscopic Display Issues
•
•
•
•
• Stereopsis
Stereoscopic Display Technology
Computing Stereoscopic Images
Stereoscopic Display and HTDs.
Works for objects < 5m. Why? Stereopsis views of the
The result of the two slightly different
external world that our laterally-displaced eyes
receive. Time-parallel stereoscopic
images
• Image quality may also be affected by
– Right and left-eye images do not match in
color, size, vertical alignment.
– Distortion caused by the optical system
– Resolution
– HMDs interocular settings
– Computational model does not match viewing
geometry. Disparity
• If an object is closer than the fixation point, the
retinal disparity will be a negative value. This
is known as crossed disparity because the two
eyes must cross to fixate the closer object.
• If an object is farther than the fixation point, the
retinal disparity will be a positive value. This
is known as uncrossed disparity because the
two eyes must uncross to fixate the farther
object.
• An object located at the fixation point or whose
image falls on corresponding points in the two
retinae has a zero disparity. Convergence Angles
f1 α+a+c+b+d = 180
β+c+d = 180
α-β = a+(-b) = δ1+δ2 =
Retinal Disparity α D1 f2 a β c δ
1 D2 b
d i δ2 Stereoscopic Display
• Stereoscopic images are easy to do badly,
hard to do well, and impossible to do
correctly. Stereoscopic Displays
• Stereoscopic display systems create a threedimensional image (versus a perspective
image) by presenting each eye with a
slightly different view of a scene.
– Time-parallel
– Time-multiplexed Time Parallel Stereoscopic
Display
Two Screens
• Each eye sees a
different screen
• Optical system directs
each eye to the correct
view.
• HMD stereo is done
this way. Single Screen
• Two different images
projected on the same
screen
• Images are polarized at
right angles to each
other.
• User wears polarized
glasses (passive glasses). Passive Polarized Projection Issues
• Linear Polarization
– Ghosting increases when you tilt head
– Reduces brightness of image by about ½
– Potential Problems with Multiple Screens (next
slide) • Circular Polarization
– Reduces ghosting but also reduces brightness
and crispness of image even more Problem with Linear Polarization
• With linear polarization,
the separation of the left
and right eye images is
dependent on the
orientation of the glasses
with respect to the
projected image.
• The floor image cannot be
aligned with both the side
screens and the front
screens at the same time. Time Multiplexed Display
• Left and right-eye views of an image are
computed and alternately displayed on the
screen.
• A shuttering system occludes the right eye
when the left-eye image is being displayed
and occludes the left-eye when the right-eye
image is being displayed. Stereographics Shutter Glasses Motion Depth Cues
• Parallax created
by relative head
position and
object being
viewed.
• Objects nearer to
the eye move a
greater distance Pulfrich Effect
• Neat trick
• Different levels of illumination require
additional time (your frame rates differ base
of amount of light)
• What if we darken one image, and brighten
another?
• http://dogfeathers.com/java/pulfrich.html
• www.cise.ufl.edu/~lok/videos/pulfrich.avi Physiological Depth Cues
• Accommodation – focusing adjustment
made by the eye to change the shape of the
lens. (up to 3 m)
• Convergence – movement of the eyes to
bring in the an object into the same location
on the retina of each eye. Summary
• Monoscopic – Interposition is strongest.
• Stereopsis is very strong.
• Relative Motion is also very strong (or
stronger).
• Physiological is weakest (we don’t even use
them in VR!)
• Add as needed
– ex. shadows and cartoons What are some new things we
can do?
•
•
•
• Lighting and Shading!
Texturing!
Stereo
Surfaces
– Normals
– Materials ...
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